In the prior art, a hot strip is generally made by heating a slab, prepared by a continuously casting method or a ingot making-bloom rolling method, heated again in a heating furnace and, subsequently, by coarsely hot-rolling and finish-rolling the heated slab. However, this process requires a slab reheating step so that it is disadvantageous in high fuel consumption. Moreover, a standby capability is made indispensable in portions of the slab yard or the heating furnace so that a large unit weight long slab has problems in its handling or maintenance. Moreover, rolling is difficult for large unit weight pieces so that it is performed for the slab at the unit of about 10 to 30 tons.
In this rolling, however, each coil has uneven portions at its leading and trailing end portions which leaves problems of product quality and production yield unsolved.
In order to solve these problems, there has been proposed a rolling process for performing the casting and hot-rolling steps completely continuously.
This completely continuous rolling process is thought to have the following advantages.
(1) The rolling and forming are performed directly from the continuous casting machine while requiring no re-heating so that fuel consumption efficiency is improved.
(2) In non-continuous rolling, each rolling material has to be smoothly gripped between the upper and lower rolls of the coarse-rolling mill group and the finish-rolling mill group so that each rolling material has to be adjusted in its leading end shape and its thickness. In completely continuous rolling, however, the adjustment is not required.
(3) In non-continuous rolling, the temperatures of the leading and trailing ends of each rolling material become lower and invite the disadvantage that the formed products are heterogeneous in qualities. However, the completely continuous casting is free from this disadvantage.
(4) In non-continuous rolling, there arise between the preceding rolling material and the succeeding rolling material the time and mechanical spaces in which the rolling actions are not made, so that the effective availability of the precious rolling mill is lowered, which degrades productivity. However, the completely continuous rolling is free from this disadvantage.
(5) In non-continuous rolling, when each rolling material is gripped at its leading end by the rough rolling group and the finish-rolling group, shocks occur so that the rolling mill has to be designed to have a strength capable of withstanding the shocks. The completely continuous rolling has little shock so that it is advantageous in the design of the strength of the rolling mill.
As described above, completely continuous rolling can advantageously solve the problems of the non-continuous rolling of the prior art. At this stage, however, the casting capacity of each continuous casting machine is far inferior to that of the rolling mill so that the process cannot be large-scale.
The following points are noted considering the existing process for making hot strip. Specifically, the steel is made at every charge (of 50 to 300 tons) by batch operation determined by the capacity of the converter or the electric furnace. In view of the order of construction of the hot strip, on the other hand, most lots of equal width and thickness are made in units of 50 to 100 tons on an average.
If a continuous rolling at the unit of at least one charge is possible, therefore, even non-continuous rolling can expect effects similar to those of the completely continuous rolling.
As based on the concept thus far described, we have previously proposed a hot strip rolling process, as disclosed in Unexamined Published Japanese Patent Application No. 59-92103, which enables a rolling capacity similar to that of the continuous rolling, although non-continuous, by dividing the rolling step into a former step and a latter step so that a plurality of rolling mills capable of performing a high draft rolling according to the rolling capacity of the downstream rolling mill are arranged at the former step, and by suitably rewinding the sheet bar, as wound in an up-end state at the former step. By developing this hot strip rolling process, productivity is remarkably improved.
In this rolling process, the slab is rolled, after being continuously cast, into such a sheet bar by the high draft rolling mill which is wide enough for later finish rolling. However, this process has a problem that the hot-rolled steel sheet products are frequently defective at their surfaces by scale biting scratches or roll marks due to the high draft rolling when the sheet bar is made so that the percentage of defective products is high. Another problem is that the work rolls of the high draft rolling mill are seriously worn on their surfaces to make it necessary to replace them so that the initial target of the rolling operation matching the rolling capacity of the downstream rolling mill cannot be achieved.
The invention has an object to provide a hot-rolling process which is improved from the aforementioned hot strip rolling process to lower the product cost more and improve product quality, and a rolling apparatus for the process.